Children go from one interest to the next with bewildering speed. This isn't a bad thing. The entire world is new to them. They don't know what they like yet. If you try to push them toward just one particular thing simply because you think they need to specialize, then you restrict their interest at a time when they most need to be expanding.
It's okay to offer help in developing an interest, especially if it is something you can do together. Take him to the library and help him find books on dinosaurs (but if some other book interests him don't try to guide him away from it). Let him tell you all about what he learns. Ask him questions that he might be able to answer.
The key strategy is to focus on him, not on any particular interest or skill. Spending time with him and making what's important to him important to you tells him that he is important. If he focuses on one subject, praise his dedication. If he focuses on a hundred things, celebrate his enthusiasm and interest. There will be plenty of time later in life, once he has an idea of what actually interests him, for you to help pursue it.